The present invention pertains to the synthesis of boron carbide containing ceramics and particularly to the use of certain nonvolatile and easily-processed precursors of such ceramics.
Boron carbide is a highly refractory material that is of great interest for both its structural and electronic properties. The boron carbide structure is composed of icosahedral units that are linked by both direct covalent bonds and by three-atom chains. The most widely accepted structural model for B4C (B12C3) has B11C icosahedra with C-B-C intericosahedral chains. However, single phase boron carbides are also known with carbon concentrations ranging from 8.8 to 20 atomic %, and localized phases of varying composition may exist in a single material. This range of concentrations is made possible by the substitution of boron and carbon atoms for one another within both the icosahedra and the three-atom chains.
Particularly important properties of boron carbide include its high-temperature stability, high hardness, high cross-section for neutron capture, and excellent high-temperature thermoelectric properties. This combination of properties gives rise to numerous applications, including uses as an abrasive wear-resistant material, ceramic armor, a neutron moderator in nuclear reactors, and, potentially, for power generation in deep space flight applications.
Various synthetic processes of a variety of forms of boron carbide are known. For example, boron carbide powders can be made by a number of reactions. One such reaction is the carbothermal reduction of boric oxides at high temperatures and high pressures. Coatings can be made by chemical vapor deposition using a gaseous mixture of a boron source (such as a boron halide, borane, or diborane) and a carbon source (such as methane or chloroform), typically in the presence of hydrogen. Recently, Zhang et al discussed the important of nanoscale boron carbide materials and demonstrated the use of plasma-enhanced chemical vapor deposition to generate boron carbide nanowires and nanonecklaces. Han et al. have also recently reported the formation of mixtures of crystalline boron carbide nanorods and boron-doped nanotubes upon the reaction of boron oxide vapor with carbon nanotubes.
As mentioned above, boron carbide has a high cross-section for neutron capture. More specifically, the 10B isotope is an effective neutron radiation absorber that degrades into relatively innocuous materials. As a result, the highly refractory ceramic boron carbide has been incorporated into emergency shutdowns balls for nuclear reactors. Subsequent findings have shown, however, that boron carbide is too sensitive to oxidation at higher temperatures. Recent efforts have been made to increase the oxidation resistance of these shutdown balls by making B4C/SiC/C composites. These materials should exhibit the oxidative stability of SiC, but still have the neutron absorbing properties of B4C. Such composites have already proven to be oxidatively stable up to 1200xc2x0 C.
Of particular interest is the formation of boron carbide ceramic fibers. Because of their low density, high strength, high temperature stability (up to 2300xc2x0 C.), high modulus of elasticity, chemical inertness, hardness and electrical conductivity, boron carbide fibers have many potential uses in harsh environments. Boron carbide fibers have previously been made by complex processes, such as the boriding of carbon fibers by their reaction with BC13 and H2. Thus, the development of a single-source precursor to such fibers would be a major advance. Previous efforts include the successful development of melt-spinnable polymeric precursors to SiC, Si3N4, BN and SiNCB ceramic fibers.
Ideally, a chemical precursor which can be processes into form such as films and fibers, including nanoscale structures, may be used to make boron carbide containing ceramics. This requires, however, the development of such precursors which can be processed. For example, once dissolved in a suitable solvent, ceramic precursors which can be processed can be applied as thin films to substrates then pyrolyzed to yield thin ceramic films. Such dissolved precursors may also be applied to substrates by various dipping or spraying techniques or could be dry spun to form polymer fibers that could be converted to ceramic fibers upon pyrolysis. Such ceramic fibers could also be made by extrusion of polymer melts to form polymer fibers, followed by pyrolytic conversion to the ceramic fiber. In addition, such precursors could be formed into molded monoliths, if desired.
Some attempts have been made to develop such precursors which can be easily processed. For example, U.S. Pat. No. 4,946,713, assigned to the assignee of this patent application, describes the use of poly(alkenylpentaborane) as a precursor to boron carbide and other ceramic materials. Due to the high reactivity of the B5H8 substituent, however, this precursor is thermolytically and hydrolytically unstable, rendering it difficult to use commercially. Seyferth has shown that decaborane-diamine and decaborane-diphosphine polymers can be pyrolyzed to form boron nitride and boron phosphide, respectively, mixed with boron carbide and graphite.
In view of the shortcomings of the prior art described above, it is desirable to develop or identify efficient, stable precursors which can be processed and which form substantially pure ceramic materials. It is also desirable for a ceramic precursor to provide, with only slight modifications to the precursor, different ceramic materials (e.g., boron carbide versus a boron carbide/silicon carbide composite) or the same ceramic material but with a range of variable chemical compositions and thus variable physical properties (e.g., boron carbide having 78.25 weight percent boron versus boron carbide having 85.4 weight percent boron). In particular, some of the characteristics of born carbide are known to vary with changes in the relative weight (or atomic) percent of boron and carbon. For example, as the atomic percent of boron increases, the neutron absorption capability of the boron carbide ceramic improves; on the other hand, as the atomic percent of carbon increases to 13.3%, the thermoelectric properties of the boron carbide ceramic improve. Likewise, the hardness of the material increases with an increasing atomic percent of carbon up to the saturation limit of 20%. Thus, it would be desirable to vary the atomic percent of boron and carbon of a boron carbide ceramic depending on the particular application and desired properties.
The present invention provides a method for making a boron carbide containing ceramic. The method involves forming a precursor comprising at least one monosubstituted decaboranyl group and at least one substituting group containing carbon and pyrolyzing the precursor to form the ceramic product. The precursor may be a molecular precursor comprising one or two decaboranyl groups bridged by a single substituting group or a polymeric precursor having a polymeric backbone of a portion of the substituting group and pendant groups each including one or more decaboranyl groups.
The method of the present invention can be used to make a variety of ceramics. For example, if the substituting group is a hydrocarbon with no other elements, the resulting ceramic product may be boron carbide. Alternatively, if the substituting group is a hydrocarbon with at least one other ceramic forming element, the resulting ceramic product may be a composite containing boron carbide and another ceramic material. In addition, the pyrolysis gas can either be an inert gas, such as argon, or a reactive gas, such as ammonia. In the event that a reactive gas is used, a ceramic composite may be formed by utilizing a ceramic forming element from the reactive gas, even if the substituting group consists solely of a hydrocarbon.
The present invention also provides a polymeric precursor for making a boron carbide containing ceramic comprising a polymeric backbone and a plurality of pendant groups having at least one monosubstituted decaboranyl group. Exemplary polymeric precursors include polyhexenyldecaborane and a copolymer formed by the copolymerization of hexenyldecaborane and allyltrimethylsilane.
The present invention also contemplates the use of the precursor of the present invention to make a variety of forms of boron carbide containing ceramics.
The precursor of the present invention can be dissolved in various organic solvents and/or melted and, in these states, can be used to prepare fibers (including melt spun fibers), coatings, and films (including spun cast films) of boron carbide containing ceramics as well as nanostructures such as nanofibers, nanocylinders, and nanoporous structures.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.